Development of the Mark 19 (Mk.19 in short) grenade machine gun was initiated by US Navy in 1966, after the initial experience gained during Vietnam war. Since about 1962, US forces in Vietnam used several multi-shot 40mm grenade launchers that fired low velocity 40x46mm M406 grenades originally developed for M79 single-shot weapon. Among these, most notable was the Mark 18 belt-fed grenade launcher, developed by Hughes corporation; this was a hand-cranked weapon thatprovided significant short-range firepower to riverine crafts and marine infantry. However, all such weapons lacked effective range, and US Navy decidedto develop a new, automatic, self-powered weapon around the high-velocity 40x53mm M384 grenade (which was originally developed for US Army's M75 automatic grenade launcher, used on UH-1 and AH-1 combat helicopters). Development of Mark 19 automatic grenade launcher began inJuly 1966, and by October 1967 firs three working prototypes of the new weapon were ready for official field trials. The production Mark 19 Model 0 grenade launchers entered service in Vietnam in early 1968, on board of riverine crafts and UH-1 Huey gunship helicopters belonging to US Navy. In around 1971, the basic design was slightly improved, and type classified as Mark 19 Model 1; these weapons were manufactured by US Naval Ordnance Station Louisville. In around 1974, US Navy attempted to produce more streamlined version of the basic design, known asMk.19 Mod.2, but this development never went past prototype stage, and in 1976 work began on simplified and more robust improved version of the Mk.19, which was type classified as Mark 19 Model 3 (Mk.19 mod.3) in 1981. The manufacturing contract for Mk.19 Mod.3 weapons was granted to Saco Defence (now General Dynamics Armament and Technical Products – GDATP). By the year 2000, at least 25 thousands of Mk.19 mod.3 grenade launchers were in service worldwide, mostly with all branches of US armed forces.

The Mark 19 Model 3 (Mk.19 mod.3) automatic grenade launcher is air cooled, belt fed, blowback operated machine gun. It fires from open bolt and uses advanced primer ignition principle to decrease peak recoil. To provide minimum of parts and maximum reliability of feeding, Mk.19 uses two-stage feed, when each round is first withdrawn rearwards from the belt on the opening stroke of the belt and then placed into T-slot cut in the bolt face; on the closing stroke, the round is already properly positioned on the bolt and is feed straight to the chamber; empty cartridge cases are ejected to the bottom as they are pushed down from the T-slot by the next cartridge (the same principle has been employed on some WW1 era machine guns). Ammunition is fed using special disintegrating belt; when rounds are fed into the chamber,links stay on the case and are ejected from the gun along with spent cartridge cases. Ammunition is usually supplied in belt boxes with capacity of 32 or 48 rounds. Loaded 32-round box weights about 19 kg (42 lbs). Mk.19 grenade launcher s fitted with dual spade grips, and can be fired in semi-automatic of full automatic modes. Alternatively, electric trigger pack can be installed forremote controlled mounts. It can be installed on standard M3 tripods (using Mark 64cradle / adapter) for infantry use, or on various vehicle mounts on jeeps,armored vehicles, naval vessels etc. By default, Mk.19 mod.3 grenade launchers are equipped with machine gun type iron sights, but other types of sightingequipment (including night sights and range-finding electronic sights) are available. Today the primary ammunition type for Mk.19 mod.3 in US service is the 40mm M430 High Explosive, Dual Purpose(HEDP) round, which can penetrate up to 50mm (2") of rolled homogenous armor and provide effective anti-personnel fragmentation effect with damage radius of about 15 meters (lethal damage / kill zone radius 5 meters).